Underground prospecting system



Oct. 27, 1953 c. A; DQNALDSQN 557380 UNDERGROUND PRosPEcTmc 'svsrmArran/vcr 54 Sheets-REU 3 Filed Jan. 9, 1950 N L mw mm V00 N vl In A i@f 5 M m n T r.. .H

n wutlam. tww. SG um. m m @Ax Patented Oct. 27, 1953 UNITED ST TES PNTQFFICE 6 Claims.

This invention relates to geophysical prospecting, and more particularlyto electrical pulse methods of geophysical prospecting.

A main object of the invention is to provide a novel and improvedtechnique for geophysical prospecting which is very simple, whichprovides rapid and accurate indications of the location and depth ofgeological formations, and which requires a minimum amount of apparatusand personnel.

A further object of the invention is to provide an improved electricalpulse technique for locating and determining the depth of geologicalformations, said technique providing an instantaneous indication at agiven station of the depth and arrangement of underground geologicalstratay in the neighborhood of the station, whereby it is possible toplot cross-sectional profiles of underground. formations along anydesired line by making readings at spaced points along said line, thetechnique being of great value in mapping geological formations and inlocating deposits of minerals, oil, and other valuable substances.

A still further object of the invention is to provide an improved methodof 4geophysical prospecting employing electrical pulses, wherein thelocations of underground strata are determined by comparing thereflections cf a pulse from the various underground formations at thelocation under test, the method providing simultaneous indications fromall of the underground formations at a single receiving station, wherebythe respective depths of the formations in the neighborhood of thereceiving station may be shown on a single indicator.

A still further object of the invention is to provide an improvedelectrical pulse technique of geophysical prospecting by thepulse-reflection method wherein very short, constant-amplitude pulses,of the order of a fraction of a microsecond, are employed, thus enablingreiiections from all the underground strata at the location under testto be presented on a single indicator, the indications being presentedin such a manner that the relative depths of the different strata may beinstantly determined.

A still further object of the invention is to provide an improvedelectrical means for determining the locations of underground geologicalformations which involves relatively simple components, which providesvery accurate readings, which requires a minimum number of operatingpersonnel, which is highly portable, and which simple to operate.

A still further object of the invention is t0 provide an improvedelectrical pulse apparatus for determining the locations of undergroundgeological formations, said apparatus being relatively inexpensive,being compact in size, providing precise indications of the relativedepths of underground formations at the location under test, and `whichinstantaneously determines said depths by a single transmission at thelocation.

Further objects and advantages of the invention will become apparentfrom the following description and claims, and from the accompanyingdrawings, wherein:

Figure l is a block diagram showing schematically one form of apparatusemployed in carrying out the method of the present invention.

Figure 2 is a partial block diagram showing another form of apparatusfor electrical-pulse geophysical prospecting by the method of thepresent invention, wherein the pulse reections are collected by the sameelectrodes used to apply the pulse to the earth.

Figure 3 is a block diagram illustrating still another form of apparatusfor electrical-pulse prospecting by the method of the present invention,wherein the pulser and the receiver are set up as separate units.

Figure 4 is a schematic wiring diagram showing the details of a pulsegenerator suitable for use as a pulser in method of the presentinvention.

Figure 5 shows the voltage wave forms present at dierent points in thepuiser circuit of Figure fl in relation to the sawtooth sweep voltageemployed with the cathode ray tube indicator of the receiver, asobtained with the apparatus of the present invention.

Figure 6 is a schematic wiring diagram showing the details of adifferentiating circuit suitable for use in the embodiment of theapparatus shown in Figure 2.

Figure 7 shows the voltage wave forms present at different points in thedifferentiating circuit of Figure 6 in relation to the wave form of theapplied pulse.

Figure 8 is a schematic wiring diagramV showing the details of apre-amplifier suitable for use in the form of the invention shown inFigure 1.

Figure 9 shows a series of oscilloscope indications taken at spacedpoints along a line of survey, said indications being obtained by themethod and means of the present invention and enabling thev locations,depths and shapes of underground formations along said -linerto beplotted.

Various methods of electrical subsurface prospecting have been proposedwhich employ impulses of electrical energy and which are based on thegeneral procedure of transmitting an electrical impulse to the earth anddetecting reflections of the impulse from surfaces of undergroundgeological formations. The methods heretofore employed have not beensatisfactory for several reasons. One of the main sources of diillcultyin the prior methods has been lack of control of the wave form of theapplied pulse, whereby clear-cut individual reflections of the pulsefrom the respective underground formations have been impossible todisplay on an oscilloscope screen. In the prior methods, for example, inthe method. employed in U. S. Patent No. 2,077,797 to Benjamin S.Melton, the pulse is applied in such a manner as to cause disturbingoscillations which may have periods of the same order as the timediierences which it is desired to measure, namely, the difference intime between that required for the pulse wave-front to travel from thetransmitter to one reilecting stratum and then to the receiver, and thatrequired for the same pulse wave-front to travel from the transmitter toanother reflecting stratum and then to the receiver. Furthermore, sincethe pulse may travel several hundred feet in amicrosecond. in order toshow up two different pulse paths differing in length by distances ofthe same order (i. e., by distances of the order of several hundredfeet), the duration of the pulse must necessarily be much less than amicrosecond in order to provide a distinct indication for each pulsepath. Thus, for example, assuming that the velocity in the earth of agiven pulse is, say, 45,000 miles per second, i. e., the pulse travelsabout 250 feet per microsecond, and assuming that a first totalreflected path of the pulse to and from a given stratum is 500 feet anda second total reflected path of the pulse to and from an adjacentstratum is '750 feet, there would be a microsecond difference in timebetween the pulse wave front indications on the receiving cathode rayoscilloscope screen. If the pulse is one microsecond or longer induration, the received indications would overlap, and therefore, the tworeflecting strata would fail to provide separate indications. This hasheretofore not been clearly recognized in the art. The present inventionis concerned with definitely limiting the duration of the pulse to arelatively small fraction of a microsecond to avoid the possibility ofoverlapping of the received reiiections.

Referring now to the drawings, and more particularly to Figures l, e, 5and 8, I I designates a pulse generator arranged to apply pulses to thesurface of the earth, shown at i2, by means of electrodes I3, I3, saidelectrodes, for example, being suitable plates spaced apart from four tosix feet and being connected by a short line to the output of the pulseril. The puiser is enersized by a suita 'le power supply it, which alsoenergizes the receiver, shown at i5. Connected to the output of receiveri5 is an oscilloscope i5 Whose sweep circuit is synchronized with thepulser Ii.

As shown in Figure l, receiving electrodes i?, Il are provided at adistance of the order of 1GO feet from the transmitting electrodes E3,I3, said receiving electrodes being connected by a short line to apre-amplifier IS. The output of the pre-amplifier la is connected by asuitable line 24 to the receiver l5.

In the above system, a high power pulse from the pulser lI having aduration of a fraction of a microsecond is applied by the plates i3, 3to the earth and travels by diiferent reflected paths. such as shown forexample at IS and 23, to the receiving plates il, Il. The path IS isobtained by reflection from the interface 2e between adjacent formations2! and 22. The path 23 is obtained by reflection from the interfacebetween formation 22 and the next subadiacent formo,- tion. A largenumber of diiferent reflective paths for the pulse therefore may betweenthe transmitting plates i3, I3 and the receiving plates Il', I'I, thelocus of each path being determined by the location of an underground.reilccting interface` f The pulse reflections are piclresl up by thereceiver plates il, Il, and, after a plication in the pre-amplifier i8,are transp; ted by the .line 2li, which is preferably a coaiial line, tothe receiver I5. After further amplication in the receiver I5, thepulses are applied to the plates of the oscilloscope i6, and the variousreflections appear as separate pips on the scllcscope trace. By taking aseries of photogra of the oscilloscope traces along a given survey line,-a proiile chart of the underground strata may be obtained, as in.Figure 9.

The pulse is preferably of the order of 1,21. microsecond in duration,and negative puise of approximately 10 kw. is employed in order toobtain reflections from a depth of 20ct feet or more.

The pulser I i is a typical hard puiser such as is employed in radar andis of standard construction, except that instead of applying the pulseto a magnetron or other radio frequency generator, the pulse is appliedto the earth. Rcferring to Figures 4.- and 5, it will bc seen that a 6C4is employed as a free-running blocking oscii-- later. This is set toprovide a 1/2 microsecond pulse at a frequency of about 2596 cycles persecond. The 6C4 tube is used to trigger the sweep generator for theoscilloscope iii by a positive pulse 2li at A The output wave il at B isused to trigger the type 367 driver tube, which is also a-line-controlled biocking oscillator set to provide a pulse 28 of about1/(1 microseeond duration at its output, as shown at 0. The output ofthe 807 tube is used` to drive the -pulse tube, shown as a type 3E29tube, biased to cut-off. The zii-ohm resistors in the plate and gridleads of the 3R29 tube are provided to prevent highfrequency parasiticoscillations. A square negative pulse 3G, shown at 17, of a duration ofabout 1/4 microsccond therefore appears across the output terminals ofthe puiser.

The receiver I5 may be merely a conventional video amplifier, or maycomprise an oscillator connected to a wide-band E. amplifier. Thepre-amplifier I8, shown in detail in Figure 8, comprises two stages ofvideo amplication with a cathode follower output stage connected to thecoaxial line 24. Line 2t is about 100 feet long and is terminated by a50-ohm resistor 25 at its connection to the receiver I5, so that nooscillations will be set up in the line.

The pre-amplifier I8 serves two purposes: it amplies the short pulsesreflected by underground strata, and drives the coaxial line 24 from thelow-impedance cathode follower. This, with the 50ohm resistor at thereceiver end of the coaxial line insures the transmission of the signalswithout any reflections from the ends o the line.

A hard tube pulser is employed at II to avoid acszeso oscillations 'duetonii'smatch'between 'the pulser "and ground. 'If a suitable 'matchingnetwork Ais "natively, ground rods, or aground 'rod and `plate for e'ach'set 'o'f 'electrodes may 'be employed.

The signal from the 'receiver 'is appliedto the oscilloscope, which, 'asshown in Figure 5, has 'a triggered -sweep 3 I, synchronized Awith theoutput 'pulse 30. If it is desired to examine particular depth, "adelayed 'and Ye'Xp'aifded 'sweep may be employed.

In the embodiment shown in Figure 2, the reflections 'are picked upbythe 'same electrodes I3, 'I3 used 'to 'apply the pulse to the earth.In this embodiment, a differentiating circuit 'and amplifier '3'2 isemployed, arranged so that only the shorts-time constant reiiections-are amplied and 1applied to the oscilloscope. This prevents theoscilloscope trace from being driven off the screen by 4long-timeconstant effects. The differentiating 'circuit is shown in detail inFigure 6. Curve zA in Figure 7 shows the appearance of the trace whenthe pulser output is connected directly to the oscilloscope. This curveis similar to the trace obtained when the pulse 30 is applied to 'acapacitor, except for the pip's 33 due to 're- 'ections By passing thewave through thesmall capacitor shunted by the low 'resistance toground, as shown in Figure 6, the long-time constant component of thewave is blocked out and only the pips due to 'the 'reectio'n's areapplied to the video amplifier. The resultant wave obtained at theloutput of the dilerentiating circuit 'is shown 'at B.

The system 'of Figure 32 'may be employed 'to make 'a rapid survey of 'alocation by `innxnti'ng the apparatus in a vehicle, lwhich may beemployed as a ground, and by employing a trailing wire as the highvoltage electrode. In this manner, an area may be surveyed without thenecessity of obtaining leases until after the survey.

In the embodiment illustrated in Figure 3, the pulser II and thereceiver I are set up as separate, physically spaced units. Thus, thepulser may be set up at a fixed location and the receiver moved to anumber of different points, a reading being taken at each point. Bymeasuring the distance between the pulser II and the receiver 1-5 ateach point, the total depth to a reflecting surface may be calculated.In this embodiment, the oscilloscope sweep synchronizing signal may beobtained from the first pulse to reach the receiver, or a separate radiofrequency circuit may be employed to transmit the synchronizing signalfrom the pulser to the receiver.

The oscilloscope screen may be calibrated in any suitable manner toprovide a depth chart such as shown at 34 in Figure 9. Said figure showsa series of traces 35 to 42 obtained from reflection tests made atsuccessive stations along a line of survey. Each trace contains spacedpips indicating the presence of interfaces between undergroundformations. By joining the corresponding pips, a rough cross-sectionalprofile map of the geological structure along the line of survey may beobtained, as shown in Figure 9.

'frire curves of Figure e represent the er au actuai survey. The tracesss tuiteinibt 'anomalous pip forms at 43 to '68, 'indicating thepresence of an unusual formation at i9. i'y actual drilling, it wasfound that the fomaton i9 was a coal seam, said seam being discovered'at a depth of 'about 1'50 feet, thereby confirming the results obtainedby the pulse survey.

While certain specific embodiments of impivcd methods and means forelectric-ar geo'physical prospecting have been disclosed 'in theforegoing description, it will be understood that various 'modificationswithin the spirit of the Yinvention may 'occur to those skilled in theart. Therefore it is intended that no lii'nitations be placed 'on theinvention except as deiined by the 'scope 'of `the appended claims.

What is 'claimed is:

1. In a Ygeophysical prospecting apparatus, the combination of a lowfrequency generator arranged to provide successive rectangular outputpulses, each having a total duration of the order of a fraction of amicrosecond, means electrically coupling said generator to the earth, avideo receiver including an oscilloscope provided with sweep means, andmeans synchronizing said sweep means with the output pulses of saidgnerator, said receiver being coupled to the earth at a distance 'fromthe generator oi the saine order as the distance between 'successiveunderground interfaces 'and being 'arranged to pick up respectivereflections of a pulse froinsuccess'ive subsurface formations, saidreceiver being further arranged to provide separate visual 'indicationsof the respective rilectins on said oscilloscope.

2. In a geophysical prospecting apparatus, 'the combination of a lowVfrequer'icy generator airanged to provide successive rectangular outputpulses, each having a total duration -of the order of a fraction of ami'crcsco'nd, means electrically coupling said generator t the earth, svideo receiver including an oscilloscope provided with sweep means, andmeans 'synchronizing said sweep means with the output pulses 'of said'genrator, said receiver being coupled to 'the earth at a distance fromthe generator of the order of feet and being arranged to pick uprespective reflections of a pulse from successive subsurface formations,said receiver being further arranged to provide separate visualindications of the respective reilections on said oscilloscope.

3. In a geophysical prospecting apparatus, the combination of a lowfrequency generator arranged to provide successive rectangular outputpulses, each having a total duration of the order of a fraction of amicrosecond, means electrically coupling said generator to the earth, avideo receiver including an oscilloscope provided with sweep means,means synchronizing said sweep means with the output pulses of saidgenerator, pickup means spaced a distance from said generator of thesame order as the distance between successive underground interfaces andelectrically coupled to the earth, and means electrically connectingsaid pickup means to the receiver, whereby reflections of a pulse fromsuccessive subsurface formations will be detected by said pickup meansand transmitted to said receiver, said receiver being arranged toprovide separate visual indications of the respective reflections onsaid oscilloscope.

4. In a geophysical prospecting apparatus, the combination of a lowfrequency generator arranged to provide successive rectangular outputpulses, each having a total duration of the order of a fraction of amicrosecond, means electrically coupling said generator to the earth, a

-video receiver including an oscilloscope provided vwith sweep means,means synchronizing said line connecting said pickup means to the re`ceiver, whereby reections of a pulse from successive subsurfaceformations will be detected -by said pickup means and transmitted tosaid receiver,l relatively low-resistance means corinectingv the ends ofsaid transmission line to the earth, and means associated with thereceiver arranged to provide separate visual indications of therespective reiiections on said oscilloscope.

5. In a geophysical prospecting apparatus, the combination of agenerator arranged to provide a rectangular output voltage pulse havinga total duration of the order of a fraction of a microsecond at arelatively low frequency, means at said generator electrically couplingthe output of said generator to the earth, a video receiver including.an oscilloscope provided with sweep means, means synchronizing saidsweep means with the output pulse of said generator', a pickup electrodecoupled to the earth and spaced from said generator a distance of theorder of the distance between successive underground interfaces, andmeans electrically connecting said electrode to the input of saidreceiver, whereby respective reflections of the pulse from successiveinter faces between subsurface formations will be detected by saidelectrode and transmitted to said receiver, said receiver being arrangedto provide separate visual indications on said oscilloscope of therespective reections, spaced in accordance vwith the diierent timeperiods required for the respective reflections to reach the electrode.

6. In a geophysical prospecting apparatus, the

combination of a generator arranged to provide a rectangular outputvoltage pulse having a total duration of the order of a fraction of amicrosecond at a relatively low frequency, means at said generatorelectrically coupling the output of said generator to the earth, a videoreceiver including an oscilloscope provided with sweep means, meanssynchronizing said sweep means with the output pulse of said generator,a pickup electrode coupled to the earth and spaced from said generatorat a distance of the order of the distance between successiveunderground inter- .faces, a transmission line connecting said elec-References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,838,371 Deardorff Dec. 29, 1931 2,077,707 Melton Apr. 20,1937 2,139,460 Potapenko Dec. 6, 1938 2,165,214 Blau July 11, 19392,172,688 Barrett Sept. l2, 1939 2,280,226 Firestone Apr. 21, 19422,423,024 Hershberger June 24, 1947 2,426,501 Hart Aug. 26, 19472,426,918 Barrett Sept. 2, 1947 2,438,836 Wolff Mar. 2o, 1948 2,498,381Smith Feb. 21, 1950 OTHER REFERENCES Preliminary Note on an AutomaticRecorder ,Giving a Continuous Height Record of the Kennelly-HeavisideLayer, by T. R. Gilliland and G. W. Kenrick; Research Paper No. 373,Reprint from Bureauof Standards Journal of Research; vol. 7, November1931. Copy in Division 51 343-5,

